This invention is in the field of measurement techniques and relates to a method and a system for measuring the parameters of patterned structures.
Techniques for thickness measurements of patterned structures have been developed. The term xe2x80x9cpatterned structurexe2x80x9d used herein, signifies a structure formed with regions having different optical properties with respect to an incident radiation. More particularly, a patterned structure represents a grid having one or more cycles, each cycle being formed of at least two different locally adjacent stacks. Each stack is comprised of layers having different optical properties.
Production of integrated circuits on semiconductor wafers requires maintaining tight control over the dimensions of small structures. Certain measuring techniques enable the local dimensions of a wafer to be measured with relatively high resolution, but at the expense of discontinued use of the wafer in production. For example, inspection using a scanning electron microscope gives measurements of the parameters of a patterned structure, but at the expense of cleaving it and thus excluding it from continued processing. Mass production of patterned structures such as wafers requires a non-destructive process for controlling thin film parameters in a manner enabling the local measurements to be performed.
One kind of the conventional techniques for measuring thickness of thin films is disclosed in U.S. Pat. No. 4,999,014. The technique is based on the use of small spot size and large numerical aperture for measurements on small areas. Unfortunately, in the case of a very small structure, this approach suffers from a common drawback associated, on the one hand, with the use of a small spot-size and, on the other hand, owing to the large numerical aperture, with the collection of high diffraction orders. The term xe2x80x9csmall spot-sizexe2x80x9d signifies the spot diameter similar in size to the line or space width of the measured structure, i.e. a single grid cycle. This leads to various problems, which are difficult to solve. Indeed, not all the stacks"" layers are in the focus of an optical system used for collecting reflected light, the optical system being bulky and complicated. Detected signals are sensitive to small details of a grid profile and to small deviations in the spot placement. Diffraction effects, which depend significantly on the grid profile and topography and therefore are difficult to model, have to be included in calculations.
Another example of the conventional techniques of the kind specified is disclosed in U.S. Pat. No. 5,361,137 and relates to a method and an apparatus for measuring the submicron linewidths of a patterned structure. The measurements are performed on a so-called xe2x80x9ctest patternxe2x80x9d in the form of a diffraction grating, which is placed in a test area of the wafer. Here, as in most conventional systems, a monochromatic incident light is employed and diffraction patterns are produced and analyzed. However, a large number of test areas are used and also information on multiple parameters cannot be obtained.
According to some conventional techniques, for example that disclosed in U.S. Pat. No. 5,087,121, portions with and without trenches are separately illuminated with broadband light, the reflection spectrum is measured and corresponding results are compared to each other with the result being the height or depth of a structure. However, it is often the case that the structure under inspection is such that the different portions cannot be separately imaged. This is owing to an unavoidable limitation associated with the diameter of a beam of incident radiation striking the structure.
The above approach utilizes frequency filtering to enable separation of interference signals from different layers. This is not feasible for layers of small thickness and small thickness difference because of a limited number of reflection oscillations.
Yet another example of the conventional technique for implementing depth measurements is disclosed in U.S. Pat. No. 5,702,956. The method is based on the use of a test site that represents a patterned structure similar to that of the wafer (circuit site), but taken in an enlarged scale. The test site is in the form of a plurality of test areas each located in the space between two locally adjacent circuit areas. The test areas are designed so as to be large enough to have a trench depth measured by an in-line measuring tool. The measurements are performed by comparing the parameters of different test areas assuming that the process is independent of feature size. For many processes in the field such as etching and photoresist development, this assumption is incorrect and this method is therefor inapplicable.
It is a major object of the present invention to overcome the above listed and other disadvantages of the conventional techniques and provide a novel method and system for non-destructive, non-contact measurements of the parameters of patterned structures.
It is a further object of the invention to provide such a method and system that enables the relatively small amount of information representative of the structure""s conditions to be obtained and successfully processed for carrying out the measurements, even of very complicated structures.
According to one aspect of the present invention, there is provided a method for measuring at least one desired parameter of a patterned structure which represents a grid having at least one cycle formed of at least two locally adjacent elements having different optical properties in respect of incident radiation, the structure having a plurality of features defined by a certain process of its manufacturing, the method comprising the steps of:
a) providing an optical model, which is based on at least some of said features of the structure and on relation between wavelength range of the incident radiation to be used for measurements and pitch of the structure under measurements, and is capable of determining theoretical data representative of photometric intensities of light components of different wavelengths specularly reflected from the structure and of calculating said at least one desired parameter of the structure;
b) locating a measurement area for applying thereto spectrophotometric measurements, wherein said measurement area is a grid cycles containing area and is substantially larger than a surface area of the structure defined by one grid cycle;
c) applying the spectrophotometric measurements to said measurement area by illuminating it with incident radiation of a preset substantially wide wavelength range, detecting light component substantially specularly reflected from the measurement area, and obtaining measured data representative of photometric intensities of each wavelength within said wavelength range;
d) analyzing the measured data and the theoretical data and optimizing said optical model until said theoretical data satisfies a predetermined condition; and
e) upon detecting that the predetermined condition is satisfied, calculating said at least one parameter of the structure.
Thus, the main idea of the present invention consists of the following. A patterned structure, whose parameters are to be measured, is manufactured by several sequential steps of a certain technological process completed prior to the measurements. Actual design-rule features can often be found in the structure in sets (e.g. read lines in memories). The term xe2x80x9cdesign-rule featuresxe2x80x9d signifies a predetermined set of the allowed pattern dimensions used throughout the wafer. Hence, information regarding the desired parameters can be obtained using super-micron tools such as a large spot focused on a set of lines.
The present invention, as distinct from the conventional approach, utilizes a spectrophotometer that receives reflected light substantially from zero-order. The zero-order signal is not sensitive to small details of the grid profile of the structure such as edge rounding or local slopes. This enables the effects associated with diffracted light not to be considered, and thereby the optical model, as well as the optical system, to be simplified. Moreover, the large spot-size enables large depth of focus that includes the whole depth of the structure to be measured. When the spot includes a number of grid cycles, then the measurement is insensitive to local defects, exact spot placement or focusing.
In the case of wafers, each such element in the grid cycle consists of a stack of different layers. The features of such a structure (wafer), which are dictated by the manufacturing process and should be considered by the optical model, may be representative of the following known effects:
specular reflection from the different stacks within the grid cycle;
interference of reflected light from layers within each stack;
dissipation within transparent stacks due to cavity-like geometry formed in the grid-like structure;
specular contributions due to width of stacks relative to the wavelength;
polarization due to the incident beam interaction with a conductive grid-like structure, if present;
effects due to limited coherence of illumination;
interference between light beams reflected from each stack within the grid cycle, taking into account the above effects.
The contribution of each of the above effects into the theoretical data are estimated in accordance with the known physical laws.
The optical model, being based on some of the features, actually requires certain optical model factors to be considered in order to perform precise calculations of the desired parameters. If information of all the features is not available and the model cannot be optimized prior to the measurements, this is done by means of a so-called initial xe2x80x9clearningxe2x80x9d step. More specifically, there are some optical model factors which, on the one hand, depend variably on all the features and, on the other hand, define the contribution of each of the existing optical effects into the detected signal. The values of these optical model factors are adjusted along with the unknown desired parameters during the learning step so as to satisfy the predetermined condition. The latter is typically in the form of a merit function defining a so-called xe2x80x9cgoodness of fitxe2x80x9d between the measured and theoretical data. The resulting optical model factors can consequently be used in conjunction with known features to enable precise calculations of the desired parameters of the structure.
Preferably, the measurement area is the part of the structure to be measured. Alternatively, the measurement area is located on a test pattern representative of the actual structure to be measured, namely having the same design rules and layer stacks. The need for such a test pattern may be caused by one of the following two reasons:
1) If the measurement area is not substantially smaller than the available surface area defined by the actual structure to be measured, then the test site is implemented so as to include an extended structure;
2) If the structure is very complicated or consists of ambiguous under-layer structure, then the test site is implemented with the same geometry as that of the actual structure to be measured, but with a simplified under-layer design, thus allowing simplified measurements of the top layers.
According to another aspect of the present invention, there is provided an apparatus for measuring at least one desired parameter of a patterned structure that represents a grid having at least one grid cycle formed of at least two locally adjacent elements having different optical properties in respect of an incident radiation, the structure having a plurality of features defined by a certain process of its manufacturing, the apparatus comprising:
a spectrophotometer illuminating a measurement area by an incident radiation of a preset substantially wide wavelength range and detecting a specular reflection light component of light reflected from the measurement area for providing measured data representative of photometric intensities of detected light within said wavelength range, wherein the measurement area is substantially larger than a surface area of the structure defined by the grid cycle; and
a processor unit coupled to the spectrophotometer, the processor unit comprising a pattern recognition software and a translation means so as to be responsive to said measured data and locate measurements, the processor being operable for
applying an optical model, based on at least some of said features of the structure and on relation between wavelength range of the incident radiation to be used for measurements and pitch of the structure under measurements, for providing theoretical data representative of photometric intensities of light specularly reflected from the structure within said wavelength range and calculating said at least one desired parameter, and
comparing said measured and theoretical data and detecting whether the theoretical data satisfies a predetermined condition.
Preferably, the spectrophotometer is provided with an aperture stop accommodated in the optical path of the specular reflected light component. The diameter of the aperture stop is set automatically according to the grid cycle of the measured structure.
Preferably, the incident radiation and the reflected light received by the detector are directed along substantially specular reflection axes.
More particularly, the invention is concerned with measuring height/depth and width dimensions on semiconductor wafers and is therefore described below with respect to this application.